We know the Covenant absorb technology, in a large part from the Forerunner. What about the new weapon first seen at Reach? Some sort of precise laser type weapon. Sounds to me like the weapons the Sentinels use. Perhaps the Covenant have just recently discovered this Halo, about, lets say a month before Reach. They could found the Sentinels on Halo, and reverse enginered them to create that weapon. It could be that the Covenant have not come across other Halos yet, or maybe the Sentinel were the type of gaurds chosen by the Forerunner for this particular Halo (or this Installation, assuming the others are not neccacarily Rings.)

We know now that the Covenant were there before us, but not how long before. Otto might have a point, here.